Double-layer metal sheet and method of fabricating the same

ABSTRACT

A method of bonding two powder components having different shrinkage factors, one of which is metal powder, is disclosed herein. In the preferred method, at step  1.1 , each powder component is separately mixed with a powder binder to substantially match the other powder component&#39;s shrinkage factor. Then at steps  1.2  to  1.4 , the powder component mixtures are compacted in a die set to form a green compact. Next at step  1.5 , the green compact is debindered and at step  1.6 , the green compact is sintered in a furnace while applying pressure on a surface of the green compact during the sintering process. In this way, the two powder components can be bonded directly without a need for an intermediate gradient zone and a double-layer metal sheet can be thus formed.

BACKGROUND AND FIELD OF THE INVENTION

This invention relates to a method for bonding two metal powdercomponents having different shrinkage factors using powder metallurgy,more particularly but not exclusively, for fabricating double layermetal parts, sheets or components.

It is widely known that formation or fabrication of a double layer metalcomponent using powder processing requires an intermediate gradientmaterial composition disposed between the two layers of metal powder.The intermediate gradient material is a mixture of the two materialpowders to be bonded to mutually compensate the respective shrinkagefactor of each material and thus provides a region or zone to allow agradual change between the two material powders when the two layers aresintered together. This alleviates deformation or warpage of the twolayers during sintering.

However, the use of the intermediate gradient zone creates extra processsteps of preparing the intermediate mixture and arranging the mixturebetween the two layers during the fabricating process which may furthercomplicate the process.

It is an object of the present invention to provide a bonding methodwhich alleviates at least one of the disadvantages of the prior artand/or provides the public with a useful choice.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a method ofbonding two metal powder components having different shrinkage factors,the method comprising the steps of:

-   -   i. mixing one of the powder components with a powder binder to        reduce the shrinkage difference between the powder components,    -   ii. forming a green compact from the powder component mixtures,    -   iii. sintering the compact to form a permanent bond between the        two powder components, and    -   iv. applying pressure on a surface of the compact during the        sintering step (iii).

An advantage of the described embodiment of the invention is that thetwo metal powder components can be bonded without an intermediategradient zone. The use of an appropriate amount of pressure during thesintering process helps to control the shrinkage of the compact andreduces or prevents warpage thereof.

Preferably, the powder binder reduces the shrinkage difference betweenthe two metal powder components such that the shrinkage factor of onemetal powder component is substantially matched to that of the othermetal powder component.

The other metal powder component may also be mixed with powder binderbefore the compacting step (ii). The powder binder for one powdercomponent may be the same or different from that used for the othermetal powder.

Preferably, one of the metal powder is mixed with three to five weight %of powder binder.

Preferably, the method further comprises a step of removing the powderbinder in a debinding furnace prior to the sintering step.Alternatively, the powder binder is removed in a sintering furnacefollowed by the sintering step.

In one embodiment, the green compact is formed in a suitably shaped dieand the method further comprises the steps of arranging one of thepowder component mixtures in the die as a base layer, arranging theother powder component mixture on top of the base layer; and compressingthe powder component mixtures together to form a desired shape of thecompact.

Preferably, the pressure applied on the surface of the compact duringthe sintering step is between 2 g/cm² and 10 g/cm². Alternatively, thepressure applied on the surface of the compact during the sintering stepmay be more than 10 g/cm². Preferably, the required pressure is producedby a ceramic plate. As an example, the ceramic plate may be placeddirectly on the surface of the compact during the sintering step toexert the required amount of pressure. As a further example, the ceramicplate may be spaced from the compact such that the space between theplate and the compact may be 50 to 100 microns.

In another embodiment, the green compact may be formed by tape castingand the method further comprises the step of mixing the powder componentmixtures with a solvent and polymer binder to form respective slurrymixtures prior to the compacting step. Preferably, the method furthercomprises the steps of layering one slurry mixture on top of the otherto form a double layer slurry and heating the double layer slurry toform the compact.

The compact may also be formed by powder rolling.

The present invention also relates to a metal part, double layer ormulti-layer plate or part produced using the method for differentmaterials.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings in which,

FIG. 1 is a flow chart depicting a preferred method for producing adouble layer component without an intermediate gradient zone usingpowder metallurgy;

FIG. 2 shows a die set used in the method of FIG. 1;

FIG. 3 shows a sintered double layer component according to thepreferred embodiment;

FIG. 4 shows a microstructure view of the double layer component of FIG.3;

FIG. 5 shows a microstructure view of another double layer componentproduced using the method of FIG. 1;

FIGS. 5 a and 5 b show microstructure views of two further double layercomponents produced using the method of FIG. 1;

FIG. 6 shows a method of applying pressure on a surface of a greencompact during sintering; and

FIG. 7 shows a tape casting apparatus for forming a double layer greencompact.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention will be described using anexample of fabricating a double layer metal plate using powdermetallurgy and FIG. 1 shows a flowchart depicting the fabrication steps.

In this embodiment, an iron (Fe) based powder material, hereinafterreferred to as powder A is used to form the first metal layer. For thesecond metal layer, nickel (Ni) powder is used. It should be apparentthat the described embodiment is also applicable to other types of metalsuch as titanium (Ti), stainless steel metal powder or other alloys.

Powders A and B have different shrinkage factors and to reduce theshrinkage difference, a powder binder, such as wax powder binder isused. As shown in FIG. 1, at step 1.1, powders A and B are separatelymixed with respective proportions of the powder binder using anexperimental blender.

In this embodiment, to achieve the desired results, typically powder Bis mixed or blended with 3 to 5 wt % wax powder binder whereas powder Ais mixed with 5 to 15 wt % powder binder. To further reduce the effectsof warping or distortion during sintering, it is preferred that layerand binder formulation is optimised at about 5 wt % binder for the baselayer (powder B) and about 7 wt % binder for the top layer (powder A).It should be apparent that the above binder and material wt. proportionsare only examples and the proportions may be varied according to thetypes of powder materials used since different material have differentshrinkage characteristics and the proportions may be determined by trialand error to obtain satisfactory results.

When both powders A and B are properly mixed, the powder mixtures arefurther processed or consolidated in a suitably shaped die to obtain aself-supporting compact or green body, and in this embodiment a die set20 shown in FIG. 2 is used to form the required shape of the doublelayer plate. The die set 20 comprises a base die 21 having a cavity 21a, an upper die 22 a and an ejector/lower die 22 b. The ejector 22 b isfirst arranged in the cavity 21 a of the base die 21 prior to the stepof consolidating the powder mixtures in the die set 20.

At step 1.2, the powder mixture including powder A, which is the baselayer, is spread evenly on the ejector 22 b to form the desiredthickness, which in this particular case is approximately 700 to 950microns. At step 1.3, the process is repeated for the powder mixturecontaining powder B as the coating or top layer with a lesser layerthickness of about 50 μm.

Next, at step 1.4, both layers are compressed to shape or compactedtogether using the upper die 22 to form a compact commonly known as agreen body 23 of a desired shape such as that shown in FIG. 2. In thisparticular case, the green body 23 has dimensions as depicted in FIG. 2.The compression force applied on the upper die 22 is about 25 tons. Thegreen body 23 is unsintered but possess sufficient green strength to beejected (using the ejector 22 b) as an integrated part from the base die21 ready for the next process step. Typically, the green body 23 isformed at room temperature. Alternatively and preferably, the green body23 may be formed at a temperature between 40° C. and 90° C.

It would be appreciated that the compacting can be performed before thepowder mixture containing powder B is layered onto powder A. This meansthat after the powder mixture containing powder A is arranged on thebase die 21, the upper die 22 is used to compress powder A first andwhen the mixed powder of powder B is subsequently arranged on top ofpowder A, the upper die 22 is used again to compact the composition.

Next at step 1.5 and prior to the sintering step 1.6, preferably thegreen body is thermally debinded to remove the binder materials at atemperature of up to 900° C. with a heating rate between 1° C. to 3° C.per minute. The holding time at the maximum temperature is about 0.5 to2.5 hours. The purpose of the debinding is to remove the binderscompletely prior to sintering, so that when the parts are sintered in aseparate furnace, there will be no contamination to the furnace and theparts from the residual binders.

At step 1.6, the green body 23 is subject to a sintering process and itis at this step the two layers are permanently bonded together to form adouble layer plate. Sintering requires heating the green body 23 in afurnace, typically a batch or continuous furnace, at below the meltingpoint of the lower melting temperature of either powder A or B. Atomicdiffusion takes place and the boundary between powders A and B “grow” tocreate a permanent and strong bond. For this case, the green body issubjected to a sintering temperature of 1200° C. with a heating rate of2° C. to 5° C. per minute. When the maximum temperature is reached, thisis held for 0.5 to 2 hours. Preferably, the sintering is performed in acontrolled environment which is typically vacuum or of inert atmosphere.

To reduce the effects of or prevent warping and distortion of the greenbody 23 a pressure of about 2 to 10 g/cm² is applied uniformly to asurface of the green body 23 during the sintering process. This pressureis achieved by placing a flat ceramic plate having a predeterminedweight so as to produce the required amount of pressure on the surfaceof the green body 23. In this embodiment, the plate has a weight of 100to 200 grams to produce a direct pressure of approximately 4.5 to 9.0g/cm². It will be apparent that the amount of pressure to be appliedvaries since different types of metal components have differentcharacteristics. Preferably, the pressure is maintained throughout theentire sintering process which will alleviate warping of the green body23.

FIG. 3 shows a sintered double layer plate 24 produced in accordancewith the method proposed in FIG. 1. FIG. 4 is a microstructure view ofthe double layer plate 24 of FIG. 3 which illustrates a chemical bondingboundary between the base layer (powder A including Fe) and the toplayer (powder B including Ni) which have been bonded together directlywithout using an intermediate gradient zone. FIG. 5 is a microstructureview of another double layer plate formed from Fe powder and 17-4 PHpowder. FIGS. 5 a and 5 b show microstructure views of further doublelayer plates formed from M2 (commercial tool steel) and Fe, and M2 andNi respectively. Please note that the microstructure drawingsillustrated herein portray photographs taken on the respectivemicrostructures in order to permit satisfactory reproduction which wouldnot be possible if the actual photographs are used. However, due to thedifficulty of illustrating such stipple rendered images, the effect ofthe drawings may not be as good as the actual photographs.

The application of a suitable amount of pressure on the green body 23during sintering is unconventional since in a traditional sinteringprocess of the green body without an intermediate gradient zone, thegreen body 23 will distort during sintering. However, the inventorsrealised that applying an appropriate amount of pressure actually helpsto reduce or prevent warpage of the green body 23 when the two powders Aand B are sintered together without an intermediate gradient zone.

In another variation, a ceramic plate 50 is supported at four corners byfour members 51 formed from green bodies of powder A or B, asillustrated in FIG. 6, so that the plate 50 is spaced from the greenbody or compact 23 by about 50 to 100 microns during sintering. In thiscase, the ceramic plate 50 is arranged to apply a pressure of more than10 g/cm² on the top surface of the compact 23 when the green body 23warps during sintering so that the pressure controls the distortion ofthe green body 23.

Although the embodiment describes forming the green compact 23 using diepressing (using a die set 20), alternative one step shaping techniquesmay be used such as tape casting or powder rolling which is preferredfor mass producing double layer plates or parts.

FIG. 7 shows a tape casting apparatus for forming a double layer greencompact according to another embodiment of the present invention. Thetape casting apparatus 40 comprises two pairs of “doctor blades” 42,44,reservoirs A and B, and a carrier tape 46 which moves from right to leftas shown by the arrow X. Similar to the first embodiment, metal powdersA and B are separately mixed with polymer binders to reduce theshrinkage difference between the two metal powders. In addition, eachmetal powder is further mixed with solvent and a small amount ofcorrosion inhibitor to minimise corrosion of the double layer componentwhen fabricated to form a slurry. The slurry can be formulated usingcommercial standard formulation such as 75-80.32% of metal powders A orB; 0.4% of Menhaden fish oil, brown Z-3; 7.23% of Xylenes; 95% of Ethylalcohol; 7.23% denatured; 2.41% of Butylbenzyl phthalate, S-160; and2.41% of Poly(vinyl butyral) grade B-98.

Similar to the first embodiment, a powder binder is also mixed in thecomposition to minimise the mismatch of the shrinkage difference due tothe different materials. The first slurry 48 comprising powder A, whichis the base layer, is received in the reservoir A and the second slurry49 comprising powder B, being the top layer, is received in thereservoir B. As the first slurry 48 is dispensed from the reservoir Adue to the relative carrier tape motion, the first pair of doctor blades42 “shears” or spreads the moving slurry 48 evenly to form the baselayer. As the base layer reaches the second reservoir B, the secondslurry 49 comprising powder B is spread on top of the base layer withthe help of the second pair of doctor blades 44 to form a double layerslurry. The double layer slurry is then dried typically by infraredlamps mounted along the moving carrier tape after the second pair ofdoctor blades 44. After drying, this forms a green compact havingsufficient green strength for further handling before the sinteringprocess.

Typically, the green compact is first cut into the desired shape andthen debinded and sintered to form the double layer plate. The sinteringprocess is similar to what has been described in the first embodiment,i.e. pressure is applied on a surface of the green compact, to preventwarping or distortion.

It will be apparent that other one-step shaping technique is similarlyapplicable, such as powder rolling.

Using the described embodiments, the present invention is able toovercome the different properties between the two metal powders forexample, melting temperature difference and shrinkage difference, toproduce a double layer plate which does not exhibit the effects ofdistortion or warping.

The described embodiment should not be construed as limitative. Forexample, the preferred embodiment describes mixing the two metal powdersseparately with different amounts of powder binders, but this may not beabsolutely necessary since it may be possible to mix one of the twometal powders with binder powder to change the shrinkage characteristicsof one and still reduce the shrinkage difference between the two metalpowders.

In addition, if powder B is required to be coated also on the other sideof the base layer (powder A), then a layer of the powder mixturecontaining powder B is first arranged on the base die 21 prior todepositing the powder mixture containing powder A. After the powdermixture containing powder A is applied on top of the powder B layer, afurther coating layer of powder B is applied on top of powder A to formanother coating.

Having now fully described the invention, it should be apparent to oneof ordinary skill in the art that many modifications can be made heretowithout departing from the scope as claimed.

The present disclosure relates to subject matter contained in SingaporeApplication No. SG 200303991-4, filed on Jul. 3, 2003, the contents ofwhich are herein expressly incorporated by reference in its entirety.

1. A method of bonding two metal powder components having different shrinkage factors, the method comprising the steps of: i. mixing one of the powder components with a powder binder to reduce the shrinkage difference between the powder components, ii. forming a green compact from the powder component mixtures, iii. sintering the compact to form a permanent bond between the two powder components, and iv. applying pressure on a surface of the compact during the sintering step (iii).
 2. A method according to claim 1, further comprising mixing the other metal powder component with powder binder before the compacting step (ii).
 3. A method according to claim 2, wherein one of the metal powder is mixed with three to five weight % of powder binder.
 4. A method according to claim 2, wherein one of the metal powder is mixed with five to fifteen weight % of powder binder.
 5. A method according to claim 1, further comprising a step of removing the powder binder in a debinding furnace prior to the sintering step.
 6. A method according to claim 1, wherein the powder binder is removed in a sintering furnace followed by the sintering step.
 7. A method according to claim 1, wherein the green compact is formed in a die.
 8. A method according to claim 7, further comprising the steps of arranging one of the powder component mixtures in the die as a base layer, arranging the other powder component mixture on top of the base layer; and compressing the powder component mixtures together to form a desired shape of the compact.
 9. A method according to claim 1, wherein the pressure applied on the surface of the compact during the sintering step is between 2 g/cm² and 10 g/cm².
 10. A method according to claim 1, wherein the pressure applied on the surface of the compact during the sintering step is more than 10 g/cm².
 11. A method according to claim 9, wherein the pressure is produced by a ceramic plate.
 12. A method according to claim 11, wherein the ceramic plate is placed directly on the surface of the compact during the sintering step.
 13. A method according to claim 11, wherein the ceramic plate is spaced from the compact.
 14. A method according to claim 13, wherein the space between the plate and the compact is 50 to 100 microns.
 15. A method according to claim 1, wherein the green compact is formed by tape casting.
 16. A method according to claim 15, further comprising the step of mixing the powder component mixtures with a solvent and polymer binder to form respective slurry mixtures prior to the compacting step.
 17. A method according to claim 16, further comprising the step of layering one slurry mixture on top of the other to form a double layer slurry.
 18. A method according to claim 17, further comprising the step of heating the double layer slurry to form the compact.
 19. A method according to claim 1, wherein the compact is formed by powder rolling.
 20. A metal part produced using the method according to claim
 1. 21. A double layer plate produced using the method according to claim
 1. 22. A method of preparing a green compact from two metal powder components having different shrinkage factors, the method comprising the steps of: i. mixing at least one powder component with a powder binder to reduce the shrinkage difference between the two metal powder components; and ii. foirming a green compact from the powder component mixtures.
 23. A method of producing a metal part from a green compact formed from two metal powder components having different shrinkage factors, the method comprising the steps of: i. sinteling the green compact to form a permanent bond between the two powder components; and ii. applying pressure on a surface of the green compact during the sintering step (i). 